Heat exchangers of the type referred to above are required for a number of processes and generally are formed from stacks of plates which are sealed around their edges with respective bars, each plate defining one or more flow passages for the respective fluid medium. The alternate passages may be traversed by the different fluid media and inlets and outlets are provided on opposite sides of the heat exchanger for the respective fluid media. Neighboring passages are thus traversed by different fluids and respective fluids have correspondingly separate inlet and outlet fittings by means of which the fluids to be subjected to heat exchange are supplied and are removed.
The processes in which such heat exchangers are involved can include, for example, the rectification of natural gas, i.e. the separation of natural gas into its components in a low temperature process. Here specifically a heat exchange is required between a two-phase mixture of a liquid and a gaseous phase and some other fluid.
In all processes in which these are the requirements, there are problems which arise from the need for uniformly distributing the two phases over the entire span of each flow passage and over the number of flow passages.
Particularly when a number of flow passages are to be traversed by the two-phase mixture, a uniform distribution of the two phases to all of these passages is difficult if not impossible with prior art arrangements.
Indeed, we have discovered that when two-phase mixtures are supplied after prior formation by a manifold to a number of such passages, an imbalance between the liquid and gaseous phases in the respective passages almost invariably develops. Several passages or several regions of several passages contain an excess of the gas component while other passages tend to contain or carry an excess of the liquid component. It is possible that these difficulties arise because of the specific gravity or density differences between the two components of the two-phase mixture. The non-uniform distribution of the two phases results in a reduction in the heat exchange efficiency.
Efforts have already been proposed to attempt to ensure uniform distribution of the two phases over the various flow paths of a heat exchanger and reference may be had to U.S. Pat. No. 3,559,722 in this connection.
In this publication the two phases are supplied separately to the heat exchanger and mix only within the heat exchanger. Each phase can thus be fed uniformly to the appropriate number of passages and distributed uniformly over the entire width of each of the flow cross sections to be traversed by the two-phase mixture.
The phases are separated at least initially by a partition. Thereafter, the two phases are mixed and can enter into heat exchange with the other fluid. The mixing device is thus a gap in the partition wall.
With this system it is indeed possible to obtain a relatively effective mixing and uniform distribution of the two phases. However, the presence of a mixing device within the heat exchanger provided with gaps in the partition walls results at a weakening of the structure. Consequently, when one operates with heat exchanging fluids which are under pressure and sometimes under considerable pressure, the heat exchanger cannot be effectively used or must in highly expensive and complex ways be reinforced so as to be able to sustain the high pressures which are employed.